An individual’s genotype is the genetic code they carry in their cells that provides information for a particular trait. Their phenotype is the visible, expressed trait, such as hair color. The phenotype depends upon the genotype but can also be influenced by environmental factors.
Genotype refers to the genetic makeup of a cell. For each individual trait (such as hair or eye color), a cell contains instructions on two alleles, which are alternative forms of the gene obtained from the mother and the father. An individual’s genotype refers to the combination of these two alleles, and can be either homozygous (the alleles are the same) or heterozygous (the alleles are different).
Phenotype refers to a trait that can be observed, such as morphology or behavior.
The genotype can be determined through genotyping – the use of a biological assay to find out what genes are on each allele.
The phenotype can be determined by observing the individual.
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An individual’s genotype includes their full hereditary information, even if it is not expressed. This information is determined by the genes passed on by the parents at conception.
An individual’s phenotype only includes expressed genes. For example, if an individual has one “brown hair” allele and one “blonde hair” allele, and they have brown hair, their phenotype only includes the expressed gene: brown hair. An individual’s phenotype can change during their lifetime, depending on which genes are expressed and how the environment affects them. For example, a young child with blonde hair can grow up to be a brunette.
The causal pathway of inheritance means that genomes are passed from generation to generation without being influenced or changed by the environment. A sexually reproducing organism receives two alleles at conception, making their genotype. When they reproduce, they pass an identical copy of one of these alleles to their offspring.
As phenotypes are influenced by environmental factors, however, they cannot be directly inherited. They are only found in the next generation if the right combination of genotype and environmental factors occurred again, and just as many different genotypes can produce the same phenotype, many different phenotypes can arise from the same genotype. Thus although identical twins have the same genotype, they can have different phenotypes.
edit Video explaining the differences
This video compares genotype vs. phenotype and further explains how the changes in genotype of an individual can affect it's phenotype.